Analyzing Carbon's Rainbow


In partnership with the Initiative for Sustainability and Energy at Northwestern (ISEN), Science in Society (SiS) will profile four innovators in the area of energy and sustainability – researchers who are harnessing the power of science and engineering to better understand and even solve some of the many challenges facing our planet. This week we feature ISEN award recipient Neal Blair, professor of civil and environmental engineering and of Earth and planetary sciences.

The carbon cycle: the many processes by which carbon is circulated between land, water, and our atmosphere, serving as the basis for life on Earth. Better understanding these processes will help researchers determine how humans are affecting this cycle as we use more and more land for agriculture, burn fossil fuels and cause other environmental stressors.

Blair studies contemporary aspects of the carbon cycle by focusing on how carbon is removed from the atmosphere by plants and stored in soils and sediments. SiS asked what he’s learned so far, and how this flow of carbon informs the bigger picture.

Neal BlairNeal BlairWhat is the carbon cycle?
When we talk about the carbon cycle, we often start with the removal of carbon dioxide from the atmosphere, almost always by plants via photosynthesis. The carbon in carbon dioxide is converted to organic carbon—plant biochemicals.

Food webs are part of the carbon cycle, because when we, or bacteria, or your favorite pet consume organic matter as food and digest it for energy, most of that material is converted back to carbon dioxide and it returns to the atmosphere. So that is one example of a carbon cycle.

On which part of the carbon cycle does your research focus?
Organic carbon can take some very convoluted paths. About 99.9 percent of it is oxidized back to CO2 by biology. But one organic carbon out of a thousand escapes oxidation and is preserved or sequestered in soils and sediments. Most of my research involves trying to figure out how that happens as well as determining what happens to the preserved material.

Why do we care at all about that tiny 0 .1 percent? Well, part of the reason we have oxygen in the atmosphere is because, over billions of years, we’ve been banking those organic molecules by storing them in sediments that are ultimately transformed into sedimentary rocks.  This is important because, when a plant photosynthesizes, it produces both organic carbon and oxygen. When we (humans and other heterotrophs) use that organic carbon to make energy, we use the oxygen as well—we’re aerobic organisms. For every organic carbon that we burn, we use an oxygen. If nature were 100 percent efficient in the oxidation of organic carbon, there would be far less oxygen in the atmosphere.

The other reason we care is that, on the very early Earth, there may have been 100 times more CO2 in the atmosphere than today.  Over the very long term, billions of years, CO2 levels in the atmosphere have dropped in part because of the sequestration of organic carbon into sediments (carbonate precipitation is an even more important process in this regard). Our fossil fuels are derived from this sequestered organic carbon, so we wouldn't have natural gas or petroleum if it weren’t for this preservation process.

The cycling of the sequestered carbon back to the atmosphere normally takes millions of years, because the rocks containing the carbon must be tectonically uplifted, weathered, and re-exposed to oxygen before it is reconverted to CO2. By burning fossil fuels, we have short-circuited the very long carbon cycle, and that’s what is causing our perturbation of the atmosphere now. We’ve accelerated a process that would normally take millions of years.

Our research has been focusing on understanding what happens to this old material when it is re-exposed to the surface, largely as a result of land use. For example, when we deforest an area for agriculture, we increase erosion rates, and that exposes material to the atmosphere that should have remained buried for a longer period of time.

We begin in the mountains where we characterize the organic matter in uplifted rocks. We then study how the rocks weather and erode, as well as how new organic matter is added from forest and grassland ecosystems. We then track the eroded rock particles (sediments) down rivers, into the ocean, and across the seafloor until they are buried again in the deep sea. This hasn’t been done before.

We’ve done this in three river systems so far—one in northern California and two in New Zealand. We’ve tracked the particles from beginning to end and looked at how the organics changed. I call what we see “Carbon’s Rainbow,” because we have organic carbon that’s derived from modern ecosystems packaged with material that is thousands of years old from soils and very ancient carbon from rocks that can literally be millions of years old.  

What is the next step for your work?
There are two steps. We have to first ask [if] we’ve studied enough systems to have a global understanding.  My guess is that we haven’t. So one step would be to investigate more rivers. There are literally tens of thousands if not hundreds of thousands of watersheds in the world, and less than one hundred have been studied well.

After that, we have to ask how humans have perturbed the set of nested carbon cycles. What does the future hold if we continue to disturb the landscape, if we continue to burn fossil fuels and the climate warms or precipitation patterns change? What does that do to this mix of [ancient and modern organic] material? We don’t know.

For example, some of this older material seems to be unreactive in certain environments, but we see evidence that it’s reactive in others. In the tropics, the preliminary data—and there’s not a lot of data on this—suggest that this old source of carbon is being reactivated and going back into the atmosphere.

On the other hand, if you go onto tectonically active margins such as the Pacific Rim, we think the erosional processes are so accelerated that the ancient carbon is going into the ocean to be reburied . The rapid burial could be a net sink of carbon from the atmosphere. So it’s complicated. The impact that climate change, or land use change, will have on the carbon cycle very much depends on where you are in the world. And that’s part of what we’re trying to sort out.

Your team has been working on some innovative ways to better analyze this preserved carbon. Can you tell me about these?
[Our] materials of interest are difficult to analyze because they’re not extractable—they won’t go into solution. Most biochemical analyses and analytical instruments require having the material in solution or in a gas phase. But most of the material that we’re interested in—and this is part of why there hasn’t been a lot of work done on it—is actually non-extractable, and it’s not volatile. Ancient rock carbon is a high molecular weight material. One has to dissolve the rock away with hydrofluoric acid to isolate it. It’s very nasty. That’s part of why it’s so old, because nothing can touch it.

We’re working on a series of methods to get at this material that hopefully will not require us to dissolve the rock away. One method is pyrolysis. It’s a flash-heating, high-temperature process that breaks the material apart into smaller molecular units; then we have something small enough to run through our instruments. The technique has been around for awhile, but we’re just on the verge, I think, of getting it work to use it to resolve carbon’s rainbow—the fresh carbon from the ancient carbon.

Your team recently started a new project, funded by ISEN, aimed at producing biofuels using soil fungi. What sparked your interest for this, and how might this work?
Our interest started because of another project. The first project, which was also funded by ISEN, was to see if soil fungi would make high molecular weight materials that might contribute to natural carbon sequestration in soils.

We started a project in collaboration with a researcher at the Chicago Botanic Garden who is an expert in soil fungi, Louise Egerton-Warburton, and she’s been helping us isolate and grow fungi. A postdoc in the group, Thea Wilson, started work with some undergraduates from the Garden to see if we could find some novel organic compounds. The preliminary studies suggested that there might be some. We’re in the process of trying to verify that now.

What was interesting about the initial results was that, when we pyrolyzed the isolates, we got a suite of hydrocarbons that looked almost like petroleum. That’s what led to the [second] ISEN project. We asked for additional seed money to investigate further whether soil fungi might make materials that could potentially be used as a biofuel.

What’s appealing about soil fungi is that they are the only organisms known that are capable of breaking down both cellulose and lignin. This is of interest because crop waste is made of exactly that material, and it is difficult to convert it into anything like ethanol or hydrocarbons for biofuel. So, if you had a biological process that could take crop waste and convert it into useable products that could go into biodiesel, that could be valuable.

Now, that’s not a novel idea. There is a small literature. There are known fungi that, as my collaborator at the Garden says, are “greasy.” They seem to make a lot of fats. What we’ll be looking for is to see if there are others, besides the few that people have discovered. Maybe they’ll have higher yields, or grow faster, be easier to grow, or be more efficient.




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